Don't use absolute dir for mkbootimg symlink #187

Job	Run time
Upload the source	3m 45s
macos (macos-11)	13m 35s
linux (opensuse/leap:latest, clang gcc11-c++, CC=clang CXX=clang++)	3m 37s
macos (macos-12)	2m 57s
linux (opensuse/tumbleweed, gcc gcc-c++)	3m 0s
macos (macos-13)	4m 4s
linux (opensuse/tumbleweed, clang, CC=clang CXX=clang++)	3m 27s
linux (archlinux:base, gcc)	37m 46s
linux (archlinux:base, clang, CC=clang CXX=clang++)	3m 21s
linux (ubuntu:20.04, gcc-10 g++-10, CC=gcc-10 CXX=g++-10)	2m 40s
linux (ubuntu:20.04, clang-11 libstdc++-10-dev, CC=clang-11 CXX=clang++-11)	3m 11s
linux (alpine, gcc, CC=gcc CXX=g++)	41s
linux (fedora:latest, gcc gcc-g++)	3m 22s
linux (fedora:rawhide, gcc gcc-g++)	3m 5s
	1h 28m 31s

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